package off describes methods for interacting with OFF files and structures
formatted as OFF files. The file loading code is modeled after Ryan Holmes'
C++ code, http://www.holmes3d.net/graphics/offfiles/
type OFF struct {
NumVertices, NumFaces, NumEdges int Vertices []Vertex Faces []Face}
OFF represents the geometric values stored in the OFF format.
Does not store color.
Todo: if we could make a structure that satisfied io.Reader,
we could have less duplicate code here.